1. Field of the Invention
The invention relates to an electric power steering apparatus that assists steering by applying the rotary force of a motor, which is driven on the basis of a detection result of a steering torque applied to a steering member, to a steering mechanism. The invention also relates to a method of controlling such an electric power steering apparatus.
2. Description of the Related Art
In an electric power steering apparatus that assists steering by driving a steering assist motor in accordance with a rotation operation of a steering member such as a steering wheel and applying power generated by the motor to a steering mechanism, steering torque applied to the steering member is detected by a torque sensor, a target value of a steering assist force is determined on the basis of the detected steering torque, and the motor is subjected to drive control in order to generate the target assist force.
A column assist type electric power steering apparatus in which a steering assist motor is attached to a midway position of a steering column that connects a steering member to a steering mechanism such that rotation of the motor is reduced in speed and transmitted to a steering shaft in the interior of the steering column via a gear reducer such as a worm gear reducer or a spur gear reducer, and then applied to the steering mechanism, exists as one form of this type of electric power steering apparatus (for example, Japanese Patent Application Publication No. 2006-298300 (JP-A-2006-298300) and Japanese Patent Application Publication No. 2000-43739 (JP-A-2000-43739)).
In this type of electric power steering apparatus, when backlash between a motor-side smaller gear and a steering shaft-side larger gear of the gear reducer is excessively large, the teeth of the smaller and larger gears, which move relative to each other within the range of the backlash, collide with each other, creating an unpleasant noise (gear rattle noise). In particular, gear rattle noise generated by the action of reverse input applied from the steering mechanism side when steering is not underway may unnecessarily dismay a driver who hears this noise. Conversely, when backlash between the smaller and larger gears is excessively small, meshing friction increases, leading to a reduction in transmission efficiency, and it is therefore important to ensure that the backlash between the smaller and larger gears is appropriate.
In response to this problem, in the related art, assembly of a gear reducer has been implemented so that backlash between the smaller and larger gears is maintained at an appropriate level but since a high degree of dimensional precision is required in the outer diameter and the tooth profile of the smaller and larger gears, and in addition, a high degree of precision is also required in the distance between the respective centers of the smaller and larger gears, processing and assembly require a large number of man-hours.
The electric power steering apparatus described in JP-A-2006-298300 is provided with a compression spring that urges a worm serving as the smaller gear such that the worm is pressed toward a meshing portion between the worm serving as the smaller gear and a worm wheel serving as the larger gear. Due to the action of this compression spring, the worm is pressed against the worm wheel, and as a result, backlash errors that occur during assembly are absorbed.
The electric power steering apparatus described in JP-A-2000-43739 is constituted such that the position of a support portion on one side of a worm can be adjusted in a contact/separation direction relative to a worm wheel, and therefore backlash errors that occur during assembly can be accommodated through positional adjustment of the support portion.
According to these constitutions, appropriate backlash can be achieved without the need for a high degree of precision in processing and assembly. However, the support structure of the smaller gear is complicated, leading to an increase in the number of components, and since additional processing is required on peripheral parts, the effect of reducing the number of man-hours involved in processing and assembly is small.